SlackBot#send() support for a callback function

[aoberoi]

Description

in order to know anything about a message after it has been sent, there needs to be a way to run a callback after the chat.postMessage() method completes.

it seems that the Response#send() (from Hubot core) method will invoke the adapter's send() method with the final item in the messages... array set to a function, if the user's script specified one.

this means that all we need to do is pass that callback function into the SlackClient#send() method and make sure it's used as the callback (we can set a default value of a noop function if not).

this support should also be extended to reply() and setTopic().

this would unlock use cases such as #91

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